Pulmonary dysfunction is the most frequent life-threatening complication of AIDS. The etiology and response to therapy has been assessed by serial bronchoscopies with biopsy in AIDS patients. A. Pneumocystis. Pneumonia has been the most frequent etiology. Histopathology of initial biopsies has been shown to be similar to non-AIDS patients, but on followup biopsy after three weeks of therapy, AIDS patients still have abundant organisms, which has not been the case with non-AIDS patients. The significance of this finding is being assessed by randomizing patients: if the biopsy at three weeks shows organisms, patients are randomized to either stop therapy or to continue therapy for an additional three weeks. The end points for evaluation are histology on followup biopsy, frequency of relapses, and autopsy results. B. Kaposi's Sarcoma. In patients with known KS of the skin or lymph nodes, KS is the sole cause of pulmonary dysfunction in 20% of cases. Diagnosis can only be unequivocally established by open lung biopsy. Radiation and chemotherapy have some role in palliating these patients. C. Cytomegalovirus. One patient with CMV pneumonia has been cured with a new investigational drug, DHPG. Additional patients are being sought. D. Nonspecific Pneumonitis. In 20-40% of cases, the etiology of pneumonitis cannot be recognized. The role of HTLV-III in causing this pneumonitis is being assessed. The significance of this project lies in determining methods to improve the quality and duration of survival in AIDS patients. This investigation has resulted in recognition that many patients have inflammation without discernible cause, an observation that will spare many patients being sent for follow-up open lung biopsies.